Electrical insulators commonly known as suspension insulators can be used individually, but usually form part of a string to support an electrical conductor from a supporting structure. Generally such a suspension insulator comprises two metal hardware members secured to opposite surfaces of a suitably contoured porcelain insulator shell, one hardware member being embedded by means of cement in a cavity in the porcelain insulator shell. The hardware members, typically an upper cap and a lower pin, are each secured by a layer of cement or other suitable material. By this arrangement the metal hardware members are separated and insulated from each other. This traditional combination of metal, porcelain and cement yields a heavy unit, generally weighing eight to thirty pounds.
Prior art suspension insulators, which include a one-piece ceramic head and shed, are easy to break during manufacture, transport, or installation. During operation the insulators suffer from vandalism, especially in those areas in which hunting is prevalent. U.S. Pat. No. 4,689,445 shows a cap-and-pin insulator which has a ceramic shed with a designed failure mode. The ceramic shed is made to fracture along specific fault lines, so as to maintain the insulation properties of the linked unit.
Glass or porcelain line insulators are at risk for surface arcing phenomenon, especially in highly polluted or coastal areas. This phenomenon is related to a damp layer of conductive polluting substance on the surface of the insulator. Leakage current dries the layer in some high-current density zones, and conditions promote the generation of electric arcs which short-circuit the dry zones. Numerous solutions have been proposed to mitigate the surface arcing phenomenon. They are generally based on the principle of providing a semiconductor zone between two electrodes so as to modify the distribution of the electric field in such a way as to make it less favorable to the generation of surface arcs.
In polluted areas there is an additional problem encountered in the region of the metal pin. Due to the action of the pollution and the leakage current which flows through the metal cap and pin, a corrosion takes place. This can lead to part failure in the metal pin, and cause the line to drop.
Because the prior art has not found an adequate solution to the surface arcing problem and the corrosion of the metal pin, there is a need to wash or clean the surface of line insulators in coastal or polluted areas. This is a process which requires the use of specialized equipment and trained staff, and includes a risk of breakage of the ceramic sheds.
It would be desirable to provide a cap-and-pin type insulation unit which is lighter than those of the prior art, resists the electrical surface phenomena associated with the prior art, and provides improved mechanical properties, while providing excellent insulation properties.